The present invention relates to a process for producing bilayer vesicles by forming mixed micelles in a colloidal solution from a bilayer-forming substance and a detergent, and removing the detergent by means of flow-through dialysis.
Substances which are capable of forming bilayers (i.e., double layers) in the aqueous phase are known, for example phospholipids, such as lecithin. These bilayers are frequently in the shape of small hollow spheres which are hereinafter referred to as bilayer vesicles.
Known processes for producing bilayer vesicles, such as subjecting bilayer-forming substances to ultrasound, injecting bilayer-forming substances dissolved in organic solvents into an aqueous medium, removing detergents from micelle solutions (i.e., solutions of mixed micelles of bilayer-forming substance and detergent) by means of gel chromatography, and conventional dialysis [compare Biochim. Biophys. Acta 457, 259-302 (1976), CRC Critical Reviews in Toxicology 6, 25-79 (1978)], produce bilayer vesicles with undesired properties. The main disadvantages of such processes are characterized by the inclusion of organic solvents in the bilayer vesicles, the degradation of the bilayer-forming substance, the formation of multi-lamellar structures and, in particular, the formation of vesicles which are nonhomogeneous in size (20 to 200 nm in diameter). Furthermore, undesired dilution effects can occur and these necessitate a subsequent concentration process. If bilayer vesicles are employed as medicament carriers and/or as pharmaceutical preparations, the resultant plasma clearance and distribution in the organs are determined above all by the homogeneity of the vesicles and the vesicle size. Multi-lamellar heterogeneous structures are rapidly absorbed, in particular, by the spleen and the liver and are no longer available to the organism as a pharmodynamically active substance [Biochim. Biophys. Res. Comm. 63, 651-658 (1975)]. The extent and course of this process, and the interaction of the vesicles at the cellular level, can be controlled by selection of suitable lipid composition and morphology (size) of the vesicles [Science, Volume 205, 1,142-1,144 (1979); Biochim. Biophys. Acta, Volume 541, 321-333 (1979)].